An oxide having a chemical composition represented by La9.33Si6O26 (to be referred to as “an LSO” hereinafter) is known as an example of oxide ion conductors, and examined to be applied to an oxygen sensor and an electrolyte film of a solid-state electrolytic fuel cell to be used at high temperatures (for example, see Patent Document 1).
The ion conductivity of this LSO has crystal anisotropy; the LSO shows a high ion conductivity in its c-axis direction. Accordingly, the ion conductivity of the LSO in the form of a polycrystal is extremely lower than the physical property value. To use the LSO as a single crystal, therefore, a single crystal growing method using, e.g., the Bridgman method or the floating zone melting method (FZ) is being examined (for example, see Patent Document 2).
Unfortunately, it is difficult for the Bridgman method or the floating zone melting method to grow large single crystals. Also, the LSO has a formula represented by LaxSi6O1.5x+12, and forms a solid solution within the range of x=8 to 10. When growing an LSO single crystal from a melt having a composition represented by (La9.33Si6O26) where x=9.33, for example, the initial crystal is richer in Si than x=9.33, so the composition of the melt gradually becomes rich in La as the crystal growth progresses. In other words, since the composition of the melt changes to an La rich composition, the composition of the grown single crystal also changes to an La rich composition. When the composition thus changes in the same single crystal, the lattice constant in the crystal changes, strains build up in the crystal, and cracking readily occurs. This also makes the growth of large single crystals difficult.
Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 11-71169
Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. 11-130595